Solar cell

From Wikipedia, the free encyclopedia
For convection cells on the sun's surface, see Granule (solar physics).

A solar cell made from a monocrystalline silicon wafer with its contact grid made from busbars
(the larger strips) and fingers (the smaller ones)
A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly
into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device
whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to
light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar
panels.
Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or
an artificial light. They are used as a photodetector (for example infrared detectors), detecting
light or other electromagnetic radiation near the visible range, or measuring light intensity.
The operation of a photovoltaic (PV) cell requires 3 basic attributes:


The absorption of light, generating either electron-hole pairs or excitons.



The separation of charge carriers of opposite types.



The separate extraction of those carriers to an external circuit.

In contrast, a solar thermal collector supplies heat by absorbing sunlight, for the purpose of either
direct heating or indirect electrical power generation from heat. A "photoelectrolytic cell"
(photoelectrochemical cell), on the other hand, refers either to a type of photovoltaic cell (like
that developed by Edmond Becquerel and modern dye-sensitized solar cells), or to a device that
splits water directly into hydrogen and oxygen using only solar illumination.

Applications
Assemblies of photovoltaic cells are used to make solar modules which generate electrical power
from sunlight, as distinguished from a "solar thermal module" or "solar hot water panel." The
electrical energy generated from solar modules, colloquially referred to as solar power, is an
example of solar energy.

Cells, modules, panels and systems
Main articles: Solar panel and Photovoltaic system

From a solar cell to a PV system. Diagram of the possible components of a photovoltaic system

8
1.9
3.8
4. By adding cells to the outside of the body. the mission time could be extended with no
.9
1.4
1.4
2. using independent MPPTs
(maximum power point trackers) is preferable. allowing light to pass while protecting the semiconductor wafers.[citation needed]
Typical PV system prices in 2013 in selected countries (USD)
United United
USD/W Australia China France Germany Italy Japan
Kingdom States
Residential
1.[3]
Russell Ohl patented the modern junction semiconductor solar cell in 1946[4] while working on
the series of advances that would lead to the transistor.8
4. Otherwise. 2014 edition[1]:15
History
Main article: Timeline of solar cells
The photovoltaic effect was experimentally demonstrated first by French physicist Edmond
Becquerel.
Solar cells are usually connected in series in modules.5
Utility-scale
2.4
4. Calvin Souther Fuller and Gerald Pearson. In 1888 Russian physicist
Aleksandr Stoletov built the first cell based on the outer photoelectric effect discovered by
Heinrich Hertz in 1887.7
1.1
2. Although modules can be interconnected to create an array
with the desired peak DC voltage and loading current capacity.2
1.3
Source: IEA – Technology Roadmap: Solar Photovoltaic Energy report. Connecting
cells in parallel yields a higher current. and are connected in parallel. In 1883 Charles Fritts built the first
solid state photovoltaic cell by coating the semiconductor selenium with a thin layer of gold to
form the junctions.Multiple solar cells in an integrated group.[6]
Solar cells gained prominence when they were proposed as an addition to the 1958 Vanguard I
satellite. though (as of 2014) individual power boxes are often supplied for each
module.9
3. shunt diodes can reduce shadowing
power loss in arrays with series/parallel connected cells.4
2.
The first practical photovoltaic cell was publicly demonstrated on 25 April 1954 at Bell
Laboratories.6
2.8
1. he built the world's first photovoltaic cell in his father's laboratory. creating an additive voltage.5
4.0
1.5
2.2
2.4
2.[5] The inventors were Daryl Chapin.9
Commercial
1. the device was only around 1% efficient. for which he received the Nobel Prize in Physics in 1921. problems such as shadow effects can shut down
the weaker (less illuminated) parallel string (a number of series connected cells) causing
substantial power loss and possible damage because of the reverse bias applied to the shadowed
cells by their illuminated partners.[2]
Albert Einstein explained the underlying mechanism of light instigated carrier excitation—the
photoelectric effect—in 1905. all oriented in one plane. Photovoltaic modules often have a sheet of
glass on the sun-facing side.
Willoughby Smith first described the "Effect of Light on Selenium during the passage of an
Electric Current" in a 20 February 1873 issue of Nature. constitute a solar
photovoltaic panel or solar photovoltaic module.7
1. however. Strings of series cells are usually handled independently and
not connected in parallel. In 1839. at age 19.

[8]
The first improvement was the realization that the standard semiconductor manufacturing
process was not ideal. These arrays consisted of 9600 Hoffman solar cells. and silicone glue between the two. As their price fell. Elliot Berman testing various solar arrays manufactured by his company. However. Elliot Berman was investigating organic solar cells. and felt that this increase in price
would make alternative energy sources more attractive. had purchased and shelved a solar navigation aid prototype
from Hoffman Electronics to protect its battery business.major changes to the spacecraft or its power systems. which became a common feature in
satellites. These effects lowered 1971 cell costs to some $100 per watt. the market leader. because space users were willing to pay for the best possible
cells. Tideland's solar-powered buoy quickly
overtook Automatic. The price was determined
largely by the semiconductor industry.
Navigation market
SPC convinced Tideland Signal to use its panels to power navigational buoys. The only significant use was in space
applications where they offered the best power-to-weight ratio. after finding that
Automatic Power. leaving no reason to invest in lower-cost. featuring large wing-shaped solar arrays.
Improvements were gradual over the next two decades. the price of the resulting
cells did as well. finding solar the most interesting.
In late 1969. He
conducted a market study and concluded that a price per watt of about $20/watt would create
significant demand. "potting" the cells. Solar Power
Corporation.[9]
Solar cells could be made using cast-off material from the electronics market. acrylic plastic on the front. The team also replaced the
expensive materials and hand wiring used in space applications with a printed circuit board on
the back.[10]
. when he joined a team at Exxon
SPC who were looking for projects 30 years in the future.[7]
Berman's price reductions
Dr. relying on the rough-sawn wafer surface. The team eliminated the steps of polishing the wafers and coating them
with an anti-reflective layer. less-efficient solutions. The group had concluded that
electrical power would be much more expensive by 2000. their move to integrated circuits in the 1960s led to the
availability of larger boules at lower relative prices. this success was also
the reason that costs remained high. In 1959 the United States launched
Explorer 6.

with wholesale costs well
under $2. Cell sizes grew as equipment became available on the surplus market.
California was the first dedicated to building solar panels. Tyco and RCA.The rapidly increasing number of offshore oil platforms and loading facilities led Arco to buy
Solar Power International (SPI).40 a watt. ARCO
Solar's original panels used cells 2 to 4 inches (50 to 100 mm) in diameter. older equipment became
inexpensive.[11]
Declining costs and breakneck speed
Swanson's law — the PV learning curve
Price history of crystalline silicon solar cells since 1977
Swanson's law is an observation similar to Moore's Law that states that solar cell prices fall 20%
for every doubling of industry capacity. Panels in the 1990s
and early 2000s generally used 125 mm wafers.[12]
Further improvements reduced production costs to under $1 a watt. Exxon.
Following the 1973 oil crisis oil companies used their higher profits to start solar firms. ARCO. since 2008 almost all new panels use 150 mm
. IBM. and were
for decades the largest producers. including General Electric.[13][14]
As the semiconductor industry moved to ever-larger boules. at below $3. Large commercial arrays
could be built. forming ARCO Solar. Shell. It was featured in an article in the British weekly
newspaper The Economist. and was in continual operation from
its purchase by ARCO in 1977 until 2011 when it was closed by SolarWorld. Amoco (later purchased by BP) and
Mobil all had major solar divisions during the 1970s and 1980s. Balance of system costs were then higher than the panels. ARCO Solar's factory in Camarillo. Motorola. Technology companies also
participated. as of 2010. fully commissioned.


An array of solar cells converts solar energy into a usable amount of direct current (DC)
electricity.200 mm. but more
recently the mono returned to widespread use. Once excited an
electron can either dissipate the energy as heat and return to its orbital or travel through
the cell until it reaches an electrode.28–0. reaching $0.
Efficiency
. These cells offer less
efficiency than their monosilicon ("mono") counterparts. director of IMEC's
organic and solar department. a drop in European
demand due to budgetary turmoil dropped prices for crystalline solar modules to about $1.62/watt by
4Q2012. poly was dominant in the low-cost panel market.32 oz) of silicon per watt of
power generation. up from just 5 gigawatts in 2005. partly from the shift to Chinese manufacturers.[16]
Theory
Main article: Theory of solar cells
The solar cell works in several steps:

Photons in sunlight hit the solar panel and are absorbed by semiconducting materials.
During the 1990s. In 2014.
Manufacturers of wafer-based cells responded to high silicon prices in 2004–2008 with rapid
reductions in silicon consumption.09[14]
per watt down sharply from 2010.
such as silicon. current cells use 8–9 grams (0. using a thin-film CdTe cell
sandwiched between two layers of glass. according to Jef Poortmans. but they are grown in large vats that
reduce cost. high-quality glass sheets to cover the panels.
First Solar was briefly the largest panel manufacturer in 2009.

An inverter can convert the power to alternating current (AC). The widespread introduction of flat screen televisions in the late 1990s and early 2000s led
to the wide availability of large. By late 2011.

Electrons are excited from their current molecular/atomic orbital.[15]
Solar power is growing at a breakneck speed. with wafer thicknesses in the neighborhood of 0. the total installed capacity of PV will
surpass 150 Gigawatts.[citation needed] Silicon panels returned to dominance via
lower prices. polysilicon ("poly") cells became increasingly popular.cells. Current flows through the material to cancel the
potential and this electricity is captured. In 2008. By the mid-2000s. Prices continued to fall in 2012.

and allowable
shadow angles.5eV.
charge carrier separation efficiency and conductive efficiency. Recombination losses make up another portion of quantum efficiency. Reflectance losses are a portion of quantum efficiency under "external quantum
efficiency". (The efficiency "limit" shown here can be
exceeded by multijunction solar cells.7. Panasonic's was the
most efficient. Grade B cells were usually between
0.[19]
In 2014. VOC ratio. or near-infrared light.
A solar cell has a voltage dependent efficiency curve. eliminating shaded
. with
an infinite number of layers. noted as the Shockley–Queisser limit in 1961.[18]
In September 2013. In the extreme. other parameters are substituted:
thermodynamic efficiency.70.
Semiconductors with band gap between 1 and 1. but also make up
minor portions of quantum efficiency.
The fill factor is the ratio of the actual maximum obtainable power to the product of the open
circuit voltage and short circuit current. so less of the current produced by the cell is dissipated in internal
losses. Resistive losses are predominantly categorized under fill factor. The overall efficiency is the
product of these individual metrics. and
fill factor.6% for a silicon solar cell.
Due to the difficulty in measuring these parameters directly. The company moved the front contacts to the rear of the panel. quantum efficiency. the corresponding limit is 86% using concentrated sunlight. This is a key parameter in evaluating performance.
Single p–n junction crystalline silicon devices are now approaching the theoretical limiting
power efficiency of 33.)
Main article: Solar cell efficiency
Solar cell efficiency may be broken down into reflectance efficiency. thermodynamic efficiency. three companies broke the record of 25. a solar cell achieved a new laboratory record with 44. VOC ratio. have the greatest
potential to form an efficient single-junction cell. and fill
factor. typical commercial solar cells had a fill factor > 0.7%.The Shockley-Queisser limit for the theoretical maximum efficiency of a solar cell. temperature coefficients. integrated quantum efficiency. as
demonstrated by the German Fraunhofer Institute for Solar Energy Systems. In
2009.7 percent efficiency.[17] Cells with a high fill factor have a low equivalent series resistance and a high
equivalent shunt resistance. VOC ratio.4 to 0.

because of its reliance on human labor.
.
Concentrated solar power offers greater economies of scale. they must be evaluated under realistic conditions.
Subsidies
Solar-specific feed-in tariffs vary by country and within countries. even if individual cells are more
costly.[21]
Installation
Consumer installation costs have reduced only slowly. installation and
subsidies. Such tariffs encourage the
development of solar power projects. manufacturing/distribution.
The cost before subsidies is stated per unit of peak electrical power.areas. In addition they applied thin silicon films to the (high quality silicon) wafer's front and
back to eliminate defects at or near the wafer surface.[20]
Reported timeline of solar cell energy conversion efficiencies (National Renewable Energy
Laboratory)
Cost
Solar power costs consist of three categories.
Manufacturing/distribution
Many costs are proportional to the panel area or land area involved. To be useful in evaluating solar costs. given its much larger size. A higher efficiency cell may
reduce the required space and so reduce the total plant cost.

China increased
market share from 8% in 2008 to over 55% in the last quarter of 2010. second and third generation cells. Bush set 2015 as the date for grid
parity in the US. Second generation cells are thin film solar cells.[25]
The price of solar panels fell steadily for 40 years. In the four years after January
2008 prices for solar modules in Germany dropped from €3 to €1 per peak watt. the point at which photovoltaic electricity is equal to or cheaper than
grid power without subsidies.Grid parity
Widespread grid parity. while others are optimized for use in space.[23][24] The Photovoltaic Association reported in 2012 that Australia had reached
grid parity (ignoring feed in tariffs). The first generation cells
—also called conventional.
Solar cells can be classified into first. likely requires advances on all three fronts.[26] In December 2012 the
price of Chinese solar panels had dropped to $0. interrupted in 2004 when high subsidies in
Germany drastically increased demand there and greatly increased the price of purified silicon
(which is used in computer chips as well as solar panels). the
commercially predominant PV technology. George W. that include amorphous
.[27]
Materials
Basic structure of a conventional solar cell and its working mechanism. Solar
cells can be made of only one single layer of light-absorbing material (single-junction) or use
multiple physical configurations (multi-junctions) to take advantage of various absorption and
charge separation mechanisms. During that
same time production capacity surged with an annual growth of more than 50%. The recession of 2008 and the onset of
Chinese manufacturing caused prices to resume their decline. These
materials must have certain characteristics in order to absorb sunlight. traditional or wafer-based cells—are made of crystalline silicon. Some cells are designed to
handle sunlight that reaches the Earth's surface.[22] In 2007 BP claimed grid parity for Hawaii and other islands that
otherwise use diesel fuel to produce electricity. Proponents of solar
hope to achieve grid parity first in areas with abundant sun and high electricity costs such as in
California and Japan.60/Wp (crystalline modules).
Solar cells are typically named after the semiconducting material they are made of. that includes materials such as polysilicon and
monocrystalline silicon.

They consist of small
crystals giving the material its typical metal flake effect. that are typically grown by the Czochralski process.
Monocrystalline silicon
Main article: Monocrystalline silicon
Monocrystalline silicon (mono-Si) solar cells are more efficient and more expensive than most
other types of cells. Despite the fact that their efficiencies had been low and the stability of the
absorber material was often too short for commercial applications.[28]
. as this approach does not require
sawing from ingots. also known as
"solar grade silicon". like an octagon. Solar cells made of c-Si are made from wafers between 160 to
240 micrometers thick. The third
generation of solar cells includes a number of thin-film technologies often described as emerging
photovoltaics—most of them have not yet been commercially applied and are still in the research
or development phase. The corners of the cells look clipped. Bulk silicon is separated into multiple categories according to crystallinity
and crystal size in the resulting ingot.silicon. Solar
panels using mono-Si cells display a distinctive pattern of small white diamonds. Polysilicon cells are the most common
type used in photovoltaics and are less expensive. building integrated photovoltaics or in small stand alone devices.
Polycrystalline silicon
Main article: Polycrystalline silicon
Polycrystalline silicon.
Ribbon silicon
Ribbon silicon is a type of polycrystalline silicon—it is formed by drawing flat thin films from
molten silicon and results in a polycrystalline structure. CdTe and CIGS cells and are commercially significant in utility-scale photovoltaic
power stations. because the wafer
material is cut from cylindrical ingots. there is a lot of research
invested into these technologies as they promise to achieve the goal of producing low-cost. the most prevalent bulk material for solar cells is crystalline silicon (c-Si).
Crystalline silicon
Main article: Crystalline silicon
By far. or multicrystalline silicon (multi-Si) cells are made from cast square
ingots—large blocks of molten silicon carefully cooled and solidified. These cells have lower efficiencies and
costs than multi-Si due to a great reduction in silicon waste. ribbon or wafer. yet less efficient than those made from
monocrystalline silicon. often organometallic compounds as well as
inorganic substances. These cells are entirely based around the
concept of a p-n junction. highefficient solar cells. Many use organic materials.

[32]
Cadmium telluride
Main article: Cadmium telluride photovoltaics
Cadmium telluride is the only thin film material so far to rival crystalline silicon in cost/watt.[citation needed] The quantum efficiency of thin film solar cells is also lower
due to reduced number of collected charge carriers per incident photon.[29]
Thin film
Main article: Thin film solar cell
Thin-film technologies reduce the amount of active material in a cell. while the outer edges are sold as conventional poly. the inner sections are high-efficiency mono-like cells (but square
instead of "clipped"). copper indium gallium selenide (CIGS) and amorphous silicon (a-Si)
are three thin-film technologies often used for outdoor applications.[30] The majority of film
panels have 2-3 percentage points lower conversion efficiencies than crystalline silicon. CdTe
cost per installed watt was $0. which achieved ~12%
efficiency. although they
have a smaller ecological impact (determined from life cycle analysis).
thin film panels are approximately twice as heavy as crystalline silicon panels. As of December 2013. this design uses polycrystalline casting chambers with small "seeds" of mono
material.
When sliced for processing. Most designs sandwich
active material between two panes of glass. release is impossible
during normal operation of the cells and is unlikely during ﬁres in residential roofs.
Traditional methods of fabrication involve vacuum processes including co-evaporation and
.
However cadmium is a highly toxic and tellurium (anion: "telluride") supplies are limited. However.[31]
Cadmium telluride (CdTe).59 as reported by First Solar. Most recently.Mono-like-multi silicon (MLM)
This form was developed in the 2000s and introduced commercially around 2009.
The cadmium present in the cells would be toxic if released. CZTS
solar cell emerge as the less-toxic thin film solar cell technology.4% as of December 2013. The lab efficiency of GaAs thin film
technology topped 28%. It has the highest
efficiency (~20%) among all commercially significant thin film materials (see CIGS solar cell). Also called
cast-mono.[33] A square
meter of CdTe contains approximately the same amount of Cd as a single C cell nickel-cadmium
battery. This production
method results in mono-like cells at poly-like prices.[33]
Copper indium gallium selenide
Main article: Copper indium gallium selenide solar cell
Copper indium gallium selenide (CIGS) is a direct band gap material. The result is a bulk mono-like material that is polycrystalline around the outsides. in a more stable and less soluble form. CIGS technology laboratory
demonstrations reached 20. Since silicon solar panels only use one pane of glass.

creating a layered cell called a tandem cell. PE-CVD) from silane gas and hydrogen gas. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very
thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD).7 eV) than crystalline silicon (c-Si) (1. Depending on the deposition parameters.[34]
Silicon thin film
Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasmaenhanced.[37] GaAs is more commonly used in multijunction
photovoltaic cells for concentrated photovoltaics (CPV. Although GaAs cells are very expensive. The top
cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom
cell in nc-Si. also called microcrystalline silicon.1 eV).
Amorphous silicon is the most well-developed thin film technology to-date.
this can yield:[35]

Amorphous silicon (a-Si or a-Si:H)

Protocrystalline silicon or

Nanocrystalline silicon (nc-Si or nc-Si:H).
Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high
open circuit voltage.sputtering. they hold the world's record in efficiency for
a single-junction solar cell at 28.[36] Nc-Si has about the same bandgap as c-Si and nc-Si and a-Si can
advantageously be combined in thin layers.8%.
Gallium arsenide thin film
The semiconductor material Gallium arsenide (GaAs) is also used for single-crystalline thin film
solar cells. which means it absorbs the
visible part of the solar spectrum more strongly than the higher energy infrared portion of the
spectrum. Recent developments at IBM and Nanosolar attempt to lower the cost by using nonvacuum solution processes. An amorphous
silicon (a-Si) solar cell is made of non-crystalline or microcrystalline silicon.
Multijunction cells
. as the industry favours efficiency over cost for space-based solar power. HCPV) and for solar panels on
spacecrafts. Amorphous silicon
has a higher bandgap (1.

making them a very rapidly advancing technology and a hot topic in the solar cell
field. despite cost
pressures.[citation needed]
Triple-junction GaAs solar cells were used as the power source of the Dutch four-time World
Solar Challenge winners Nuna in 2003.[38] Each layers has a different band gap energy to allow it to absorb electromagnetic radiation
over a different portion of the spectrum. are increasing sales.
. 6N and 7N) and germanium. and GaInP
2. germanium metal prices have risen
substantially to $1000–1200 per kg this year. researchers at University of Cambridge discovered that using pentacene increases
efficiency up to 95%. Those materials include gallium (4N. typically
using metalorganic vapour phase epitaxy. these products are critical to the entire substrate
manufacturing industry. and boron oxide. this is a solution-processed hybrid organic-inorganic tin or lead halide
based material. 2005 and 2007 and by the Dutch solar cars Solutra
(2005). gallium indium phosphide (GaInP). but are now used effectively with terrestrial solar concentrators.[40] Between December 2006 and December 2007.[42] Perovskite solar cells are also forecast to be extremely cheap to scale up.Dawn's 10 kW triple-junction gallium arsenide solar array at full extension
Main article: Multijunction photovoltaic cell
Multijunction cells were originally developed for special applications such as satellites and space
exploration. A triple-junction cell.[citation needed]
Research in solar cells
Pentacene
In 2014. Efficincies have increased from below 10% at their first usage in 2009 to over
17% in 2014. Twente One (2007) and 21Revolution (2009).[41]
Perovskite solar cells
Main article: Perovskite solar cell
Perovskite solar cells are solar cells that include a perovskite-structured material as the active
layer. arsenic (4N. series connected. Most commonly. Multijunction cells
consist of multiple thin films.[39]
Tandem solar cells based on monolithic.
gallium arsenide (GaAs). for example. On 15 October 2012. the cost of 4N gallium metal rose
from about $350 per kg to $680 per kg. 6N and 7N
Ga).
GaAs based multijunction devices are the most efficient solar cells to date. Additionally. Ge. making them a
very attractive option for commercialisation. pyrolitic boron nitride (pBN) crucibles for
growing crystals.
triple junction metamorphic cells reached a record high of 44%. and germanium (Ge) p–n junctions. may consist of the
semiconductors: GaAs. each essentially a solar cell grown on top of each other.

fluoroindate glasses have low phonon energy and have been
proposed as suitable matrix doped with Ho3+
ions. In bulk it should be
significantly less expensive than older solid-state cell designs. the
energy transfer upconversion process (ETU). occurs when two low-energy infrared photons are
absorbed by rare-earth ions to generate a (high-energy) absorbable photon. which can be liquid or solid. The upconverter material could be placed below the solar cell to
absorb the infrared light that passes through the silicon. Yb3+
. with the potential for lower processing costs than those used for bulk solar
. The excited ion emits light above the Si bandgap that is absorbed by the solar cell and
creates an additional electron–hole pair that can generate current. called upconversion. The photogenerated electrons
from the light absorbing dye are passed on to the n-type TiO
2 and the holes are absorbed by an electrolyte on the other side of the dye. its
price/performance ratio may be high enough to allow them to compete with fossil fuel electrical
generation. Two Er3+
ions that have absorbed this radiation can interact with each other through an upconversion
process. Er3+
ions absorb solar radiation around 1. as compared to approximately 10 m2/g of flat single crystal).54 µm. However. As example.[44]
Light-absorbing dyes
Main article: Dye-sensitized solar cells
Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate
manufacturing equipment. taking advantage of their luminescence to convert infrared radiation to visible
light. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate
titanium dioxide to greatly amplify the surface area (200–300 m2/g TiO
2. The circuit is
completed by a redox couple in the electrolyte. the increased
efficiency was small. In addition. This type of cell
allows more flexible use of materials and is typically manufactured by screen printing or
ultrasonic nozzles. This process.
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing
material.Liquid inks
In 2014.[43]
Upconversion
One efficiency technique is to incorporate lanthanide-doped materials (Er3+
. Er+
ions have been the most used. Ho3+
or a combination). researchers at California NanoSystems Institute discovered using kesterite and
perovskite improved electric power conversion efficiency for solar cells. so they can be made in a DIY fashion. Useful ions are most commonly found in
the trivalent state. consists in successive transfer processes between
excited ions in the near infrared. DSSC's can be engineered into
flexible sheets and although its conversion efficiency is less than the best thin film cells.

fabricated with crystallite
sizes small enough to form quantum dots (such as CdS. large scale production. Sb
2S
3.). QD's size quantization
allows for the band gap to be tuned by simply changing particle size. etc.[45]
Quantum dots
Main article: Quantum dot solar cell
Quantum dot solar cells (QDSCs) are based on the Gratzel cell. instead of organic or organometallic dyes as light absorbers. such as polyphenylene vinylene and small-molecule
compounds like copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes
and fullerene derivatives such as PCBM.[51]
Organic/polymer solar cells
Main articles: Organic solar cell and Polymer solar cell
Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic
semiconductors including polymers. the dyes in these cells also suffer from degradation under heat and UV light and
the cell casing is difficult to seal due to the solvents used in assembly. and practical devices are essentially
non-existent.[46]
In a QDSC. offering the possibility of a simple roll-to-roll
printing process. This TiO
2 layer can then be made photoactive by coating with semiconductor quantum dots using
chemical bath deposition. The efficiency of QDSCs has increased[47] to over 5% shown for both liquid-junction[48]
and solid state cells. very low. Current cell efficiencies are. electrophoretic deposition or successive ionic layer adsorption and
reaction. However.
They can be processed from liquid solution. but employ low band gap semiconductor nanoparticles. PbS. In addition. CdSe. They also have high
extinction coefficients and have shown the possibility of multiple exciton generation. Konarka Power Plastic reached efficiency of 8. or dye-sensitized solar cell
architecture. The electrical circuit is then completed through the use of a liquid or solid redox
couple.1%. the Prashant Kamat research
group[50] demonstrated a solar paint made with TiO
2 and CdSe that can be applied using a one-step method to any conductive surface with
efficiencies over 1%.[49] In an effort to decrease production costs. However.
much like in a DSSC. The first commercial
shipment of DSSC solar modules occurred in July 2009 from G24i Innovations.[citation needed]
. a mesoporous layer of titanium dioxide nanoparticles forms the backbone of the cell. however.
Energy conversion efficiencies achieved to date using conductive polymers are very low
compared to inorganic materials.cells.3%[52]
and organic tandem cells in 2012 reached 11. potentially leading to inexpensive. these
cells could be beneficial for some applications where mechanical flexibility and disposability are
important.

following the
same approach. one electron donor and one
electron acceptor.The active region of an organic device consists of two materials.[59][60]
Adaptive cells
Adaptive cells change their absorption/reflection characteristics depending to respond to
environmental conditions. the
concentrated light moves along the surface of the cell. unlike most other solar cell types. sometimes in the form of bulk heterojunctions. separating when the exciton
diffuses to the donor-acceptor interface. At the part of the cell where the light is most intense.[53]
In 2011. typically in the donor
material. allowing the light to penetrate the cell. They used block
copolymers. That surface switches from reflective to
adaptive when the light is most concentrated and back to reflective after the light moves along. the charges tend to remain bound in the form of an exciton. The short exciton
diffusion lengths of most polymer systems tend to limit the efficiency of such devices.[54][55]
Researchers at UCLA more recently developed an analogous polymer solar cell. researchers announced polymer cells with some 3% efficiency.[61]
Manufacture
.
Nanostructured interfaces. The system also included an array of fixed
lenses/mirrors to concentrate light onto the adaptive surface. An adaptive material responds to the intensity and angle of incident
light. the cell surface changes from
reflective to adaptive. self-assembling organic materials that arrange themselves into distinct layers. As the day continues. MIT and Michigan State researchers developed solar cells with a power efficiency close
to 2% with a transparency to the human eye greater than 65%.[61]
In 2014 a system that combined an adaptive surface with a glass substrate that redirect the
absorbed to a light absorber on the edges of the sheet. The other parts of the cell remain
reflective increasing the retention of the absorbed light within the cell.
In 2013. can improve
performance. flexible cells can be produced in bulk at a low cost and could be used to create
power generating windows. achieved by selectively absorbing
the ultraviolet and near-infrared parts of the spectrum with small-molecule compounds. When a photon is converted into an electron hole pair. that is 70% transparent and has a 4% power conversion efficiency.[56][57][58] These
lightweight. The
research focused on P3HT-b-PFTBT that separates into bands some 16 nanometers wide.

Silicon nitride has gradually replaced titanium dioxide as the preferred material.Early solar-powered calculator
This section needs additional citations for verification. The wafers are usually lightly p-type-doped. and a grid-like metal contact made up of
fine "fingers" and larger "bus bars" are screen-printed onto the front surface using a silver paste.
A full area metal contact is made on the back surface. (June 2014)
Solar cells share some of the same processing and manufacturing techniques as other
semiconductor devices. Please help improve this
article by adding citations to reliable sources. However. Unsourced material may be challenged
and removed. Such surfaces were first applied to single-crystal silicon. lowering costs. This forms a p–n junction a few hundred
nanometers below the surface.
because of its excellent surface passivation qualities. Usually this
contact covers the entire rear. like anti-reflection coatings.
Anti-reflection coatings are then typically applied to increase the amount of light coupled into
the solar cell. the stringent requirements for cleanliness and quality control
of semiconductor fabrication are more relaxed for solar cells. Some solar cells
have textured front surfaces that.
Polycrystalline silicon wafers are made by wire-sawing block-cast silicon ingots into 180 to 350
micrometer wafers. A surface diffusion of n-type
dopants is performed on the front side of the wafer. typically aluminium. A layer several hundred nanometers thick is applied using PECVD. It prevents carrier recombination at the cell
surface. followed by
multicrystalline silicon somewhat later.
The rear contact is formed by screen-printing a metal paste.
. increase the amount of light
reaching the wafer. though some designs employ a grid pattern. The paste is then fired
at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon.

the solar cells are interconnected by flat wires or metal ribbons.
. and
assembled into modules or "solar panels". and a polymer encapsulation on the back. After the metal
contacts are made.Some companies use an additional electro-plating step to increase efficiency. Solar panels have a sheet of tempered glass on the
front.

In the following paragraphs. A solar cell structure is shown in figure 1 and a solar
panel configuration in figure 2. and then raises an electron to a
higher energy state. 2013 by Shivananda Pukhrem
Since a solar cell is the only generator in a solar PV system. Silicon
is one such material that uses such process.
.How Solar Cells Work — Components &
Operation Of Solar Cells
May 13th. and then the flow of this high-energy electron to an external circuit. with reference links for better understanding.
A solar cell: A solar cell is a solid-state electrical device (p-n junction) that converts the energy
of light directly into electricity (DC) using the photovoltaic effect. a simple introduction of a solar cell and how it
operates is discussed. The process of conversion
first requires a material which absorbs the solar energy (photon). it is one of the most important parts
in a solar PV system.

known as the “light-generated
current. on average. Due to this joining. as detailed in table 1. for a length of time equal to the
minority carrier lifetime before they recombine. The carriers are separated by the
. Electron-hole pairs will
generate in the solar cell provided that the incident photon has an energy greater than that
of the band gap. Crystalline (Wafer-Based) and Thin-Film Photovoltaic Cell
A p-n junction: It is formed by joining p-type (high concentration of hole or deficiency of
electron) and n-type (high concentration of electron) semiconductor material. An animated ideal flow at short circuit is shown at
this link: http://www.
A light-generated current: Generation of current in a solar cell.PV cells: PV cells are most commonly made of silicon. Collection of these carriers by the p-n junction prevents this recombination by using a pn junction to spatially separate the electron and the hole. while movement of holes to the n-type side exposes
negative ion cores in the p-type side.
1. Absorption of incident photons to create electron-hole pairs. However.
crystalline and thin-film cells. Movement of electrons to the p-type side exposes
positive ion cores in the n-type side.
excess electrons from n-type try to diffuse with the holes of p-type whereas excess hole from ptype try to diffuse with the electrons of n-type. electrons (in the p-type material). If the carrier recombines. resulting in an electron field at the junction and forming the
depletion
region.org/pvcdrom/pn-junction/formation-pn-junction.org/pvcdrom/solar-cell-operation/light-generated-current.
Table 1.
2. then the lightgenerated electron-hole pair is lost and no current or power can be generated.
An
animated
visual
explanation
is
shown in
this
link: http://www.pveducation. and holes (in the n-type
material) are meta-stable and will only exist.” involves two important processes.pveducation. and come in two common varieties.

With this basic idea of the operation of a solar cell.pveducation. An animated explaination of the photovoltaic effect is
shown at this link: http://www. Under short circuit conditions. if the solar cell is short-circuited).e.
Voltage is generated in a solar cell by a process known as the “photovoltaic effect.
Photovoltaic effect: The collection of light-generated carriers does not by itself give rise to
power generation.org/pvcdrom/solar-cell-operation/photovoltaic-effect.” The
collection of light-generated carriers by the p-n junction causes a movement of electrons to the ntype side and holes to the p-type side of the junction. where it is now a majority carrier. If the light-generated minority
carrier reaches the p-n junction. In order to generate power. the carriers
exit the device as light-generated current.action of the electric field existing at the p-n junction. a thorough explanation of modeling of a solar
cell by using a diode with the diode ideality factor and the operation temperature as well as the
parasitic resistance (due to manufacturing defects) will be discussed later..
. it is swept across the junction by the electric field at the
junction. If the emitter and base of the solar cell are
connected together (i. then the light-generated
carriers flow through the external circuit. a voltage must be generated as well as a current.

If
we wish to have more current.Understanding The Technology Behind Solar
PV Systems
February 12th. thin-film
technology. 2013 by Shivananda Pukhrem
Solar panel by Petur (some rights reserved)
A simple solar photovoltaic (PV) system consists of the following basic components:
1. single-junction cell. etc. A solar cell is
categorized as a multi-junction cell. A solar cell: A solar cell is a solid-state electrical device (p-n junction) that converts the
energy of light directly into electricity (DC) using the photovoltaic effect. or some of type of emerging PV cell (like organic cells.
2. crystalline cell. Solar module: Collections of a single solar cell are termed as solar modules. Solar cells
are connected by a wire either in parallel or in series according to the desired output. the cells are connected in series. and if we wish to have
more voltage.
.). the cells are connected in parallel.

communication between each component is a
must.3. as mostly seen in rooftop installation. Power cable: There are two types of power cables. Solar module array: When solar modules are connected either in series or in parallel
according to the desired output. Combiner box: The combiner box is an electrical distribution box where the DC circuit
breakers are placed. Switch gears / protection: Switch gears are one of the most important components in a
PV system. over-voltage. Through this controller. switching faults. which leads to a reduction in generated output power. However. This inverter
also uses the MPPT (maximum power point tracker) technique for better output.
4.
such as over-current. which is normally either 12 volt or 24 volt. The
output of the combiner box is connected to the charge controller or the inverter. the collection of the solar module is termed a solar
module array.
9. and inverter to the transformer)
and medium-voltage power cables (transformer to the substation).
5.
8. Due to cabling. The
advantages of an such inverter are that it has more reliability with better performance. the battery life is elongated and maintains a constant
output voltage for the battery input. Solar charge controller: If we wish to charge the generated DC power from the solar
array to a battery. It converts the
generated DC power to AC power using a micro inverter for each module.
7. Low-voltage power cables (solar
panel to the combiner box. lightening faults. Solar array inverter: This inverter is also termed as a central inverter or string inverter. there is a higher chance for interference in the
power signal.
etc.
It converts the single DC output power from the combiner box into AC power which then
can be connected to the power/electric grid. combiner box to the inverter. Solar module inverter: This inverter is installed at the modular level. A central monitoring computer allows checking the communication between each
component and the execution of tripping the fault.
10.
And also it reduces the PID effect. temperature rises. It uses the
PWM (pulse width modulator) or MPPT (maximum power point tracker) technique for
better charging performance. It allows protection of the components from any fault during the operation. Due to the use of power electronic
. For such cases.
6. the problem of
partial shading or shadowing — technically termed as potential induced degradation
(PID) of the solar panel — is prominent with the use of such an inverter. For better reliability of the system.
a metal mesh encasing the cables improves shielding. a solar charge controller is
inevitable. This inverter uses the MPPT (maximum
power point tracker) technique for better optimized output.
depending on the type of system installed. Power quality issues: This has become one of the major topics in the utility sector for
common consumers. The combiner box combines the multiple DC inputs coming from the
panel (either module or array) terminations and converts these into one DC output.

is fundamental in making this energy source more cost-efficient and competitive. But the electrical properties of an insulated cell will not
allow it to generate sufficient energy to make a standard voltage electric device work (12. common mode (CM) disturbances are prominent.
Sunlight comprises photons or energy particles of different wavelengths.
.
which act as insulation at low temperatures and as conductors when exposed to heat or light
energy. flat wafers manufactured using semiconducting materials.
PHOTO V PROCESS:
The photovoltaic process converts sunlight into electricity using photovoltaic cells. they may be reflected away. when metal contacts are placed above and below
the photovoltaic cell. Thus. can be extracted for external use. Because of CM
disturbances.
The increased conversion performance. the cells have to be interconnected with other cells. their energy is transferred to the electrons of the cell atoms. which is best suited to
enhancing the efficiency of the modules.
At present. When these hit a
photovoltaic cell. When
photons are absorbed. A
thorough understanding of EMC related to the PV system is important for
better performance and high reliability for that given PV system.
The current generated by the flow of electrons. encapsulated and mounted
on a framework structure. then.switches in inverters. over 90% of the solar cells are made out of silicon.
These photovoltaic cells are thin. transience in the power is seen and the output power is distorted. Solar cells. 24 or
48 volts). With this new
energy. ie. may cut through it or be absorbed into it. are the essential
component in photovoltaic systems. the electrons escape from the orbit associated with the atoms and become part of a
current in an electrical circuit. to make up the photovoltaic module. a semiconductor metaloid
combining the properties of metal with those of insulating materials. the proportion of sunlight that the cell transforms into
electric energy.

. The ideal flow at short circuit is shown in the animation below. if the solar cell is short-circuited). prevents this recombination
by using a p-n junction to spatially separate the electron and the hole. If the light-generated minority
carrier reaches the p-n junction. The first process is the absorption of incident photons to create electron-hole
pairs. the the light-generated carriers flow through the external
circuit. known as the "light-generated current".
A second process. it is swept across the junction by the electric field at the junction. If the carrier recombines. Electron-hole pairs will be generated in the solar cell provided that the incident photon has
an energy greater than that of the band gap. and
holes (in the n-type material) are meta-stable and will only exist. the collection of these carriers by the p-n junction. involves two
key processes. If the emitter and base of the solar cell are connected together
(i. on average. electrons (in the p-type material). The carriers are separated
by the action of the electric field existing at the p-n junction.
where it is now a majority carrier. then the
light-generated electron-hole pair is lost and no current or power can be generated. for a length of time
equal to the minority carrier lifetime before they recombine.e. However.Light Generated Current
‹ Solar Cell Structure Collection Probability ›
The generation of current in a solar cell. \
.

.

.

The photovoltaic effect
‹ Spectral Response IV Curve ›
The collection of light-generated carriers does not by itself give rise to power generation. Under short circuit conditions. then the
collection of light-generated carriers causes an increase in the number of electrons on the n-type
side of the p-n junction and a similar increase in holes in the p-type material. Minority carriers cannot
cross a semiconductor-metal boundary and to prevent recombination they must be collected by
the junction if they are to contribute to current flow. the reduction of the electric field increases the
diffusion current. Under
open circuit conditions. the forward bias of the junction increases to a point where the lightgenerated current is exactly balanced by the forward bias diffusion current.
However. if the light-generated carriers are prevented from leaving the solar cell. A new equilibrium is reached in which a voltage exists across the p-n junction. and the net current is
.
The current from the solar cell is the difference between IL and the forward bias current.The ideal short circuit flow of electrons and holes at a p-n junction. there is no build up of charge. a voltage must be generated as well as a current. The collection of light-generated
carriers by the p-n junction causes a movement of electrons to the n-type side and holes to the ptype side of the junction. In
order to generate power. thereby reducing the net electric field. as the
carriers exit the device as light-generated current. This separation of
charge creates an electric field at the junction which is in opposition to that already existing at
the junction. Since the electric field represents a barrier to
the flow of the forward bias diffusion current. Voltage is generated in
a solar cell by a process known as the "photovoltaic effect".

The voltage required to cause these two currents to balance is called the "open-circuit
voltage". The following animation shows the carrier flows at short-circuit and open-circuit
conditions.
.zero.

the light-generated carriers forward bias the junction. short-circuit current and opencircuit voltage conditions. there is no net current from
the solar cell at open circuit.e.
. the minority carrier concentration on either side of the junction is increased and the
drift current. thus increasing the diffusion
current. Since the drift and diffusion current are in opposite direction. in the dark) both the diffusion and drift current are small.Simulation of carrier flows in a solar cell under equilibrium. Note the different magnitudes of currents crossing the junction. which depends on the number of minority carriers. is increased. In
equilibrium (i. Under short circuit
conditions. Under open circuit
conditions.

Movement of electrons to the p-type side exposes positive ion cores in the n-type side
while movement of holes to the n-type side exposes negative ion cores in the p-type side.
resulting in an electron field at the junction and forming the depletion region. On the n-type side. Similarly. electrons diffuse from the n-type side to the p-type side. However.
. negative ion cores
are exposed. An electric field Ê forms between the positive ion cores in the n-type material and
negative ion cores in the p-type material. On the p-type side. they leave behind
exposed charges on dopant atom sites. If the electrons and holes were not charged. which are fixed in the crystal lattice and are unable to
move. A
"built in" potential Vbi due to Ê is formed at the junction.
Since the n-type region has a high electron concentration and the p-type a high hole
concentration.Formation of a PN-Junction
‹ Drift P-N Junction Diodes ›
Overview
1. in a p-n
junction. this
diffusion process would continue until the concentration of electrons and holes on the two sides
were the same. This region is called the "depletion region" since the
electric field quickly sweeps free carriers out. as happens if two gasses come into contact with each other.
3. as shown below. A voltage results from the electric field formed at the junction.
P-n junctions are formed by joining n-type and p-type semiconductor materials.
2. positive ion cores are exposed. holes flow by
diffusion from the p-type side to the n-type side. hence the region is depleted of free carriers. The animation below shows the
formation of the Ê at the junction between n and p-type material. Joining n-type material with p-type material causes excess electrons in the n-type
material to diffuse to the p-type side and excess holes from the p-type material to diffuse
to the n-type side. when the electrons and holes move to the other side of the junction.